A polymer sustained alignment (PSA) type liquid crystal display device has a polymer structure formed in a cell in order to control a pretilt angle of liquid crystal molecules, and has been developed as a liquid crystal display element due to high-speed responsiveness and high contrast.
Manufacturing of a PSA type liquid crystal display element is performed by injecting a polymerizable composition containing a liquid crystal compound and a polymerizable compound between substrates, and polymerizing the polymerizable compound in a state where the liquid crystal molecules is aligned by applying a voltage to the polymerizable composition to fix the alignment of the liquid crystal molecules. As the cause of image persistence which is a display defect of the PSA type image liquid crystal display element, impurities and change in alignment of the liquid crystal molecules (change in pretilt angle) are known.
The image persistence due to impurities is caused by a polymerizable compound that remains due to incomplete polymerization and by a polymerization initiator that is added to promote the progress of polymerization. Therefore, it is required to minimize the residual amount of the polymerizable compound after polymerization and to suppress the amount of the polymerization initiator to be added. For example, when a large amount of a polymerization initiator is added in order to fully perform the polymerization, the voltage retention rate of a display element is reduced by the residual polymerization initiator, so as to have negative effects on display quality. Further, when the amount of a polymerization initiator used in the polymerization is reduced in order to suppress the reduction of a voltage retention rate, a polymerizable compound remains due to incomplete polymerization, and the occurrence of image persistence by the remaining polymerizable compound cannot be avoided. Further, in order to completely cure the polymerizable compound with a small amount of the polymerization initiator so as to suppress the remaining of the polymerizable compound, there is a method of applying a large amount of energy by applying strong ultraviolet rays for a long time in the polymerization. However, in this case, an increase in size of a manufacturing apparatus and a decrease in manufacturing efficiency are incurred, and simultaneously a deterioration of a liquid crystal material due to ultraviolet rays occurs. Accordingly, in a conventional liquid crystal composition containing a polymerizable compound, it has been difficult to reduce both the residual amount of an uncured polymerizable compound and the residual amount of a polymerization initiator.
Meanwhile, as another cause of occurrence of image persistence, a change in pretilt angle of liquid crystal molecules in the liquid crystal composition containing a polymerizable compound is known. That is, when a polymer, which is a cured product of the polymerizable compound, is flexible and a display element is constituted thereof, if the same pattern continues to be displayed for a long time, the structure of the polymer is changed, and, as a result, the pretilt angle is changed. Since the change in the pretilt angle is a cause of image persistence, a polymerizable compound for forming a polymer having a rigid structure in which the polymer structure is not changed is required.
In the related art, in order to prevent the image persistence by improving the rigidity of a polymer, configuring a display element using a polymerizable compound which includes a structure such as a 1,4-phenylene group consisting of a ring structure and a polymerizable functional group (referring to PTL 1) and configuring a display element using a polymerizable compound having a biaryl structure (referring to PTL 2) have been considered.
Further, in order to prevent the image persistence by improving the rigidity of a polymer, configuring a display element using a mixed liquid crystal composition of a bifunctional polymerizable compound and a tri or higher functional polymerizable compound such as dipentaerythritol pentaacryalte or dipentaerythritol hexaacryalte (referring to PTL 3) has been proposed.
In PTL 4 as other technologies, there has been disclosed a polymerizable compound-containing liquid crystal composition, in which the crosslink density of the resulting polymer is increased by allowing a polymerizable compound to have three or more polymerizable functional groups in the molecule of the polymerizable compound, liquid crystal alignment regulating force is increased after polymerization, and thus a defect such as image persistence does not occur.